39-i PLANT MORPHOLOGY 



The nature of the endosperm in angiosperms is very confusing. In 

 gymnosperms it is obviously vegetative tissue of the female gametophyte, 

 and thus necessarily arises before fertilization. In angiosperms it arises 

 after fertilization, ordinarily from a triple fusion of nuclei, one of which is 

 male, another female (since it is sister to the egg nucleus), and a third 

 vegetative. Some have regarded endosperm as gametophyte tissue stim- 

 ulated to develop by nuclear fusions. Others have considered it to be 

 sporophyte tissue, the twin of the embryo. Since it is ordinarily triploid, 

 however, it cannot be either gametophyte or sporophyte in the strict 

 sense of the terms. It might better be regarded as undifferentiated tissue 

 continuing the growth of the female gametophyte and stimulated to 

 develop by nuclear fusions. The union of the male nucleus with the polar 

 nuclei cannot be regarded as an act of fertilization because (1) the effect 

 of the fusion is merely to fvu-nish a growth stimulus; (2) more than a single 

 male and female nucleus is involved; and (3) the product of the triple 

 fusion is not a new individual. 



Embryo. The development of the embryo from the fertilized egg does 

 not, as in nearly all gymnosperms, begin with free-nuclear division, but 

 each division is accompanied by the formation of a cell wall. Since 

 embryogeny differs in dicotyledons and monocotyledons, except in the 

 earliest stages, a representative example of each will be described. 



Capsella. The sequence of embryonic stages can be followed easily in 

 the common shepherd's-purse (Capsella), a dicotyledon belonging to the 

 Cruciferae. Here the zygote, by a series of transverse divisions, gives rise 

 to a proembryo of varying length (Fig. 337A, B). The terminal cell (the 

 one farthest from the micropyle) forms practically all the embryo, while 

 the other cells give rise to the suspensor. The basal cell of the suspensor 

 is much larger than the others. The terminal cell undergoes three suc- 

 cessive divisions, each at right angles to the preceding one, thus resulting 

 in the formation of octants (Fig. 337C, D). The first division is always 

 vertical but the second vertical and the horizontal divisions may occur in 

 either order. Of the eight cells now constituting the embryo, the upper 

 tier of four cells eventually gives rise to the cotyledons and stem tip, the 

 basal tier to all the hypocotyl except its tip. 



The suspensor elongates, becoming 8 to 10 cells in length and pushing 

 the embryo downward. A peripheral layer of primary epidermal cells, 

 the dermatogen, is now cut oE by periclinal walls appearing in all 8 cells of 

 the embryo (Fig. 337^). Additional longitudinal and transverse divi- 

 sions occur in the inner cells and soon the periblem, comprising the cells 

 eventually to produce the cortex, is differentiated from the plerome, which 

 gives rise to the stele (Fig. 337F). The plerome is complete at the tip of 

 the hypocotyl but the periblem and dermatogen are not. They are com- 

 pleted at the expense of the adjacent cell of the suspensor. This divides 



